Born on 6 November 1969 in Warsaw.
He graduated from the Faculty of History at the University of Warsaw and completed postgraduate studies in public administration management at the Kozminski University of Entrepreneurship and Management in Warsaw.
He started his professional career in the Foundation for Polish-German Reconciliation (Fundacja 'Polsko-Niemieckie Pojednanie'). In 1998, he became Director of the Department of Judicature in the Office of Veterans and Opressed Persons, where he worked until 2004. In the years 2004-2006, he was the Head of the Registry for the Office of the Capital City of Warsaw; later, he served as Deputy Mayor of Warsaw's Śródmieście district.
Between January 2006 and November 2007, Jacek Sasin acted as the First Deputy Voivode of Mazovia, and later as the Voivode of Mazovia.
From December 2007, he had the honor of cooperating very closely with the late President Lech Kaczyński – first as his advisor, and on 26 November 2009 he was appointed Secretary of State and Deputy Chief of the Chancellery of the President of Poland. He resigned on 5 July 2010, when the National Electoral Commission announced the results of the presidential election.
In 2010 , Sasin became a councillor of the Mazovian Voivodship Parliament (Sejmik Województwa Mazowieckiego).
In the 2011 parliamentary elections, he was elected as a Member of the 7th Term of the Polish Parliament (Sejm), where he held the office of Vice-Chairman of the Local Self-Government and Regional Policy Committee and was a member of the Internal Affairs Committee.
In the 2015 parliamentary elections, he once again obtained the mandate of a Member of the 8th Term of the Polish Parliament (Sejm). He served as Chairman of the Public Finances Committee and of the Standing Subcommittee for the Local Government System, and was a member of the Local Government and Regional Policy Committee.
On 9 January 2018, he was appointed Secretary of State at the Chancellery of the Prime Minister, Chairman of the Standing Committee of the Council of Ministers, and later also Secretary of the Council of Ministers.
Between 4 June 2019 and 15 November 2019, he held the office of Deputy Prime Minister.
As a result of the 2019 parliamentary elections, he became a Member of the 9th Term Sejm. On 15 November 2019, was appointed as Deputy Prime Minister and Minister of State Assets.